Field of the Invention
The present invention relates to a process for producing sodium carbonate, particularly to a process for producing sodium carbonate, wherein trona is used as a starting material.
Discussion of the Background
As a typical example of natural sodium carbonate resource, trona is known which contains sodium sesquicarbonate (Na.sub.2 CO.sub.3.multidot.NaHCO.sub.3.multidot.2H.sub.2 O) as the main component. For example, it is known that there is a large trona bed containing sodium sesquicarbonate in an amount of about 90 wt % in Green Liver, Wyo., U.S.A. Further, trona beds have been discovered also in e.g. Kenya, Tanzania, Turkey and China.
As a method for producing sodium carbonate from trona, it is most common to employ a method so-called a monohydrate crystallizing method. According to this method, firstly, trona is fired at an atmosphere temperature of from 400 to 800.degree. C., whereby a sodium hydrogencarbonate content is converted to sodium carbonate, and at the same time, organic substances are decomposed and removed, to obtain crude sodium carbonate. This crude product is dissolved in water, then impurities are filtered off for purification, water is evaporated, and sodium carbonate monohydrate (Na.sub.2 CO.sub.3.multidot.H.sub.2 O) is crystallized, separated and dried to obtain sodium carbonate. This method has a feature that it is thereby possible to obtain anhydrous sodium carbonate containing little organic substances and having a large bulk density (so-called dense ash).
Heretofore, as a method for mining trona, it has been common to employ a method of digging it out directly from the ground by means of a large size mining machine in the same manner as for e.g. coal. However, by this method, the cost required for mining constitutes as much as 30 to 40% of the cost for the production of sodium carbonate. Recently, attention has been drown to a mining method so-called a solution mining method in order to reduce the cost. The solution mining method is a system wherein water is poured into a trona bed, and the sodium sesquicarbonate component in the trona bed is pumped up in the form of a mixed solution of sodium carbonate and sodium hydrogencarbonate.
However, by the solution mining method, the alkali concentration in the pumped up solution is low, and it is necessary to evaporate a large amount of water from the solution in order to obtain the desired sodium carbonate. Therefore, as a method for efficiently recovering the alkali content from the solution having a low alkali content, there is a method wherein the solution is used to dissolve crude sodium carbonate in the monohydrate crystallizing method, or a method wherein sodium carbonate decahydrate (Na.sub.2 CO.sub.3.multidot.10H.sub.2 O) is crystallized in a low temperature range. In either method, a sodium hydrogencarbonate component and other impurities are contained in the solution obtained by dissolving trona, and it is necessary to remove such impurities.
As a method for converting the sodium hydrogencarbonate component to sodium carbonate, a method of reacting sodium hydroxide, calcium hydroxide or the like, a method of blowing steam thereto to discharge carbon dioxide gas, or a method of precipitating and separating it as sodium sesquicarbonate or sodium hydrogencarbonate, is, for example, known. However, by such a method, a large amount of sodium hydroxide or calcium hydroxide was required, and consumption of energy was substantial.
For example, in a case where milk of lime is added to a trona solution having the composition as identified in Table 1 (% means wt %) to have the sodium hydrogencarbonate content reacted and removed, followed by evaporating water to precipitate sodium carbonate monohydrate, 90 kg of milk of lime as calculated as CaO is required per ton of sodium carbonate produced.
TABLE 1 Na.sub.2 CO.sub.3 14.0% NaHCO.sub.3 4.50% NaCl 0.40% Na.sub.2 SO.sub.4 0.90%
To overcome the above drawback, U.S. Pat. No. 5,283,054 discloses a carbon dioxide diffusion method. According to this method, an aqueous solution containing sodium carbonate and sodium hydrogencarbonate is heated to change a part of the sodium hydrogencarbonate to sodium carbonate, and then an aqueous sodium hydroxide solution is added in an amount sufficient to convert the rest of the sodium hydrogencarbonate to sodium carbonate, followed by cooling to precipitate and separate sodium carbonate decahydrate. This sodium carbonate decahydrate is dissolved in water and then precipitated this time as sodium carbonate monohydrate, and finally anhydrous sodium carbonate is obtained.